Implement batch_update and batch_merge for var and stddev operator #1546

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

We need to change stddev and var to use batch method for more efficient calculation.

Describe the solution you'd like

Implement batch_update and batch_merge for stddev and var.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Please find the discussions in #1525